The present invention relates to polymerization of olefins in a solution process. More specifically, the invention relates to recovery techniques for removing the polymer products from the polymer solution.
Solution polymerization of monomers containing ethylenic unsaturation is used to prepare many polymers, including polyethylene, polypropylene and a variety of copolymers. Existing solution polymerization units were designed and constructed at a time when catalyst activity was low compared to currently available catalysts. Many such polymerization units were originally designed to have adequate polymer recovery units for each reactor, or each series of reactor stages, based on the polymer yields from catalysts available at the time. Ziegler-Natta catalysts, such as those described in U.S. Pat. Nos. 4,314,912, 4,250,288, 4,319,011, and 4,547,475, provide higher yields of improved polymers such as DOWLEX(trademark) linear low density polyethylene (LLDPE) polymers, which are available from The Dow Chemical Company.
Polymer recovery units are known in the prior art. U.S. Pat. No. 4,686,279 (equivalent to EP-A-0 102 122) discloses a device on a process of recovering polymer from solution. In particular, solutions obtained from the polymerization of ethylene, with or without one or more xcex1-olefins having at least three carbon aroms, can be directly fed into the evaporating zone of the device. The melted polymer mass is then passed on to an extruder and a resulting polymer melt may be obtained whose content of volatile components may be lower than 500 parts per million.
Another polymer recovery design is taught in German patent DE-A-21-16-939 wherein a solution obtained from the polymerization of ethylene is heated, flashed to concentrate the solution, reheated, and fed into a finishing device.
Conversion of the solution polymerization process from using Ziegler-Natta catalysts to more advanced catalysts may not increase polymer production rates because of the limited capacity of the polymer recovery units. Absent additional polymer recovery units, the solution polymerization units must be operated below reactor capacity. Furthermore, the cost of an additional polymer recovery unit may not be justified in a competitive market.
Considerable research has been devoted to improving the profitability of existing solution polymerization units by providing more controllable catalysts to produce more valuable ploymers. For example, the single site constrained geometry catalysts described in U.S. Pat. Nos. 5,470,933, 5,556,928, and 5,512,693 provide polymers having substantially improved properties. Success with new catalysts can improve profitability of existing solution polymerization units. However, it would be desired to enjoy the higher activity of the catalysts without adding additional polymer recovery units.
An existing solution polymerization unit for making polyolefins, e.g., ethylene homopolymers and interpolymers, propylene homopolymers and interpolymers, etc., is shown in FIG. 1 (Prior Art). While FIG. 1 may be applied to other polyelefins, for simplicity it is discussed herein with respect to polyethylene. The existing unit includes a first stage continuous stirred tank reactor (CSTR) 10 and a second stage CSTR 12. Raw materials, including solvent, ethylene, 1-octene, trace amounts of hydrogen, and catalyst, are fed to the first stage reactor 10 through one or more feed lines 14. An interim polymer solution produced in the first stage reactor 10 then passes from the first reactor 10 to the second reactor 12 through a connecting line 16. The interim polymer solution typically has a temperature below about 200xc2x0 C. and a polymer content from 3 percent to 16 percent by weight.
The polymer solution from the second stage reactor 12 typically has a temperature below about 220xc2x0 C. and a polymer content from 3 percent to 24 percent by weight. The polymer typically comprises polyethylene or copolymers of ethylene and other xcex1-olefin monomers. A second feed line 18 optionally feeds raw materials, for example, ethylene, xcex1-olefin, solvent, hydrogen, etc., to the second stage reactor 12 to raise the polydispersity of the polymer product. The weight average molecular weight of the polymer ranges from 2,000 to 1,000,000, occasionally more, and, typically, in the case of polyethylene, 2,000 to 500,000. The polydispersity of the polyethylene ranges from 1.5 to 10.0.
The polymer solution from the second stage reactor 12 flows through a discharge line 20 to a first stage heat exchanger 22, and then flows through an entry line 24 to a first stage polymer finishing unit such as a devolatilization vessel 26. A polymer product exits the devolatilization vessel 26 through a product line 30 and gaseous monomer and vapor exit the vessel 26 through a recycle line 28. Devolatilization could occur in one or more stages although only a single stage is shown in the drawings. Alternatively, polymer recovery could be achieved by one or more hot water washes. Prior to entering the heat exchanger 22, the polymer solution is heated only by the exothermic polymerization reaction, that is the heat of polymerization. Heating and devolatilization of the polymer solution during polymer recovery can be done in one or more stages to maintain the polymer temperature below 260xc2x0 C.
To minimize polymer degradation which leads to gel formation, the polymer temperature exiting the devolatilization vessel 26 is preferably less than 200xc2x0 C. Furthermore, unreacted raw materials exiting the devolatilization vessel 26 through the recycle line 28 must be cooled for recycling.
Using Ziegler-Natta catalysts, such as TiCl3/MgCl2 catalysts, in the solution polymerization unit of FIG. 1, results in increased polymer yield until the heat exchangers 22 run at maximum capacity for polymer recovery. Additional heat exchangers could be added, however, additional cooling would also be required for cooling of recycled raw materials. Moreover, the polymer would have to be heated to higher temperatures or remain at high temperatures for a longer period of time, and would result in greater polymer degradation.
Replacing Ziegler-Natta catalysts with metallocene catalysts or constrained geometry catalysts in the solution polymerization unit of FIG. 1 improves profitability of the unit by making a more valuable product although at a lower yield than could be achieved with the Ziegler-Natta catalysts. However, the yield remains high in comparison to original design criteria such that the heat exchangers 22 still run at maximum capacity and are still a bottleneck in the process. Thus, profitability could be further improved by increasing polymer recovery capacity to match increases in polymer production capacity.
Therefore, there is a need for an improvement to existing solution polymerization units which provides for increased polymer recovery capacity to increase reactor utilization. Ideally, the improvement would increase the energy efficiency of the process without requiring a substantial redesign of the polymer recovery units. It would be desirable if such an improvement could be retrofitted with minimum capital cost or included in future construction of this and other solution polymerization units.
The present invention improves the polymer recovery capacity of solution polymerization units by flashing a polymer solution exiting polymerization reactors to produce a concentrated polymer solution having a reduced temperature and a polymer content from 10 percent to 40 percent by weight. The concentrated polymer solution can be prepared without the addition of thermal energy and can be devolatilized in conventional polymer recovery units without increasing the risk of gel formation.
One aspect of the invention provides a solution polyolefin process, comprising the steps of: polymerizing one or more olefins in a sufficient amount of solvent to produce a polymer solution having a polymer content from 3 percent to 24 percent by weight, preferably 6 percent to 18 percent by weight, wherein the polymer solution comprises a polymer having a weight average molecular weight from 2,000 to 1,000,000 and a temperature of at least about 150xc2x0 C.; flashing the polymer solution, without preheating, to produce a concentrated polymer solution having a reduced temperature and a polymer content from 10 percent to 40 percent by weight; and finishing the concentrated polymer solution. This process provides adiabatic flashing of the polymer solution using heat from the polymerization reaction, which reaction may occur in more than one stage. The adiabatic flashing of the polymer solution preferably occurs at a temperature above the crystallization temperature of the concentrated polymer solution, preferably at least about 20xc2x0 C., and more preferably at least about 30xc2x0 C. above the crystallization temperature. The process is particularly suited for the polymerization of ethylene and/or ethylene and one or more alpha-olefins and/or dienes.
Another aspect of the invention provides a solution polyolefin process, comprising the steps of: polymerizing one or more olefin monomers in a sufficient amount of solvent in a first stage reactor to produce an interim polymer solution having an interim polymer content less than about 16 percent by weight; polymerizing the interim polymer solution in a second stage reactor to produce a polymer solution having a polymer content from 3 percent to 24 percent by weight, preferably between 6 percent and 18 percent by weight, wherein the polymer solution comprises a polymer having a weight average molecular weight from 2,000 to 1,000,000 and a temperature of at least about 150xc2x0 C., preferably between 180xc2x0 C. and 230xc2x0 C. adiabatically flashing the polymer solution, without a preheat, to produce a concentrated polymer solution having a solids content from 10 percent to 40 percent by weight; and devolatilizing the concentrated polymer solution.
The flash is preferably positioned at the point of highest temperature exiting the reactors. Therefore, in the aspect of the invention comprising at least a first stage reactor and a second stage reactor, the flash can be positioned between the reactor stages. Such positioning of the flash is preferred if the first stage reactor operates at a higher temperature than the subsequent reactor stages. Additionally, positioning the flash between reactor stages is preferred if a purpose of the process is to have a higher polymer reaction concentration during the later reactions such as when the desired end product is a polymer with a relatively large amount of long chain branches (for example, in excess of about 1 long chain branch in 1000 carbon atoms). Furthermore, the flash may be positioned after each reactor in which sufficient temperature is generated. Such multiple flash steps allow the achievement of higher polymer content in the solution.
In yet another aspect of this invention, the flash can occur in the reactor itself. This flash can be augmented by the addition of a gas, such as ethylene, and this in turn allows the operation of the flash at a temperature lower than about 150xc2x0 C. but preferably greater than about 50xc2x0 C.
The olefin monomers may be introduced to both the first stage reactor and the second stage reactor.